Aug 26, 2010

A diversity of respiratory pigments have evolved in various animal taxa. One example, haemocyanin, found in arthropods and many molluscs, has copper as its oxygen–binding component, coloring the blood bluish. The respiratory pigment of almost all vertebrates and a wide variety of invertebrates is the protein haemoglobin, contained in the erythrocytes of vertebrates.

Haemoglobin consists of four subunits, each with a cofactor called a heme group that has an iron atom at its centre. The iron binds the O2; thus, each haemoglobin molecule can carry four molecules of O2. Like all respiratory pigments, haemoglobin must bind O2 reversibly, loading O2 in the lungs or gills and unloading it in other parts of the body

This process depends on cooperation between the subunits of the haemoglobin molecule. Binding of O2 to one subunit induces the others to change shape slightly, with the result that their affinity for O2 increases. And when one subunit unloads its O2, the other three quickly unload too as a shape change lowers their affinity for O2.

Cooperative O2 binding and release is evident in the dissociation curve for haemoglobin.

Over the range of O2 partial pressures (PO2) where the dissociation curve has a steep slope, even a slight change in PO2 cau2ses haemoglobin to load or unload a substantial amount of O2. Notice that the steep part of the curve corresponds to the range of O2 partial pressures found in body tissues. When cells in a particular location begin working harder—during exercise, for instance—PO2 dips in their vicinity as the O2 is consumed in cellular respiration. Because of the effect of subunit cooperativity, a slight drop in PO2 is enough to cause a relatively large increase in the amount of O the blood unloads.

As with all proteins, haemoglobin′s conformation is sensitive to a variety of factors. For example, a drop in pH lowers the affinity of haemoglobin for O2, an effect called the Bohr shift. Because CO2 reacts with water, forming carbonic acid (H2CO3), an active tissue lowers the pH of its surroundings and induces haemoglobin to release more O2, which can then be used for cellular respiration.

Carbon Dioxide Transport

In addition to its role in oxygen transport, haemoglobin also helps transport CO2 and assists in buffering—that is, preventing harmful changes in blood pH. Only about 7% of the CO2 released by respiring cells is transported in solution in blood plasma. Another 23% binds to the multiple amino groups of haemoglobin, and about 70% is transported in the blood in the form of bicarbonate ions (HCO3−). Carbon dioxide from respiring cells diffuses into the blood plasma and then into the erythrocytes.

The CO2 first reacts with water (assisted by the enzyme carbonic anhydrase) and forms H2CO3, which then dissociates into a hydrogen ion (H+) and HCO3−. Most of the H+ attaches to haemoglobin and other proteins, minimising the change in blood pH. The HCO3− diffuses into the plasma. As blood flows through the lungs, the process is rapidly reversed as diffusion of CO2 out of the blood shifts the chemical equilibrium in favor of the conversion of HCO3− to CO2.